This invention relates to furnaces for thermally cracking hydrocarbons. More particularly, the invention relates to a furnace and process for cracking hydrocarbons wherein a particularized arrangement of radiant heating tubes is employed.
It has long been known to thermally crack hydrocarbons to produce olefins and other lighter hydrocarbon products.
Typically, a thermal cracking furnace is comprised of a firebox containing a plurality of radiant heating tubes, each tube being formed into a U-shaped coil form, that extend through the volume of the firebox. A hydrocarbon feedstock is introduced into the cracking furnace through an inlet leg of a radiant heat tube and during transit through the tube is elevated by radiant heating of the tube to high temperatures, e.g. 1600xc2x0 F. during flow of the hydrocarbon from the inlet leg to an outlet leg of that furnace tube whereupon a cracked gas product is formed that is routed by the outlet leg of the tube to a quenching system which quenches the hot reaction gas to a lower temperature to yield cracked products. Unfortunately, the nature of the thermal cracking process also causes coke and tar to form along with desired hydrocarbon products. From the beginning of the practice of thermal cracking, fouling of the furnace tubes resulting from coke and tar generation has been a serious problem. When the coiled furnace tubes are fouled by coke and tar, the cracking furnace must be taken out of service to clean or replace the tubes.
As thermal cracking technology has advanced, a trend to high severity cracking has occurred in order to achieve either improved yields or increased selectivity to the desired ultimate hydrocarbon product. As a result, thermal cracking furnaces having small diameter, short length furnace tubes in the form of U-shaped coils were developed for high severity cracking to attain higher olefin selectivity. However, practice has shown that under high severity cracking conditions the coking problem becomes even more pronounced.
The conventional wisdom now prevailing in thermal cracking is that with short residence times, high severity cracking will produce the highest selectivity and olefin yield. However, under high severity cracking conditions the coking problems increase and the operational run length consequently decreases, causing shorter effective operational ability and curtailed equipment life.
Maximization of olefin output, defined as the product of average cracking cycle yield and average furnace availability, can be achieved over the long run by a furnace and process that uses the maximum available radiant heat.
The present invention provides a particular arrangement of the inlet and outlet legs of the plural radiant heating tubes of a furnace which maximizes the use of available radiant heat within the firebox and minimizes fouling of the tube coils resulting from coke and tar formation during thermal cracking. The present invention provides a furnace with a maximum utilization of radiant heat and with a minimization of local coking problems within the tubes of the furnace.
The present invention provides a furnace and process that relies on a multiplicity of radiant heating tubes, each in the form of a U-shaped coil, that are mounted within a furnace firebox such that an inlet leg of any one of the plural tubes is immediately adjacent to and spaced apart from an outlet let of another one of the plural tubes within the firebox of a thermal cracking furnace. This spacial pairing of an inlet leg of one tube with an outlet leg of another tube of the plural radiant heating tubes of the cracking furnace maximizes utilization of the available radiant heat within the firebox of a thermal cracking furnace.
To these ends, a furnace has been developed with a radiant heating zone fired by any combination of wall and floor burners and having a common external manifold from which a preheated hydrocarbon feedstock is distributed for flow to and through the plural furnace tubes. The radiant heating tube assembly for the furnace comprises a plurality of U-shaped radiant heating tubes the inlet legs of which are communicatable with the common inlet manifold, the inlet leg of each tube being located within the firebox of the furnace and extends throughout the firebox volume to a point at which the tube coils to form a vertical U-shaped section to yield a tube outlet leg which extends throughout the firebox volume in a direction opposite that of its respective inlet leg, with the outlet leg of each such tube extending to a point terminating outside of the firebox for connection to a quench exchanger system. The plural furnace tubes, each comprising an inlet and outlet leg which communicate with one another through the U-shaped coil section of the tube, are positioned and fixed with respect to one another such that within the firebox of a furnace an inlet leg of any one of the plural tubes is immediately adjacent to and spaced apart from an outlet leg of another one of the plural furnace tubes. This inlet-outlet leg pairing between the plural radiant heating tubes permits of a more uniform spacing between the legs of the plural tubes within the firebox while minimizing the occurrence of localized thermal gradients within the firebox which would detract from the uniformity of thermal conditions therein and/or create spots of localized overheating at points along the firebox flow length of a tube. This more uniform spacing between the legs of the plural furnace tubes within the firebox further provides for an optimum exposure of the exterior surface area of the inlet legs of all of the plural furnace tubes to the radiant heating surfaces within the firebox volume of the furnace and thus maximizes the utilization of the available radiant heat within the firebox of the furnace. This provides for a greater thermal efficiency for operations of the furnace to a given degree of severity of cracking and/or selectivity of conversion of hydrocarbon feedstock to the desired ultimate product, particularly olefin products.
The process proceeds by delivering preheated hydrocarbon feedstock to a common external manifold for equilibration of temperature and pressure of the feedstocks and thereafter from the common external manifold such preheated feedstock is passed by venturi control to an inlet leg of each of the plural furnace tubes to flow therethrough to and through the U-shaped coil section of the tube to the outlet leg of the tube, during which time the feedstock becomes heated to a high temperature and cracks to form a reaction product gas which exits the furnace by flow through the outlet leg of a tube to a quench exchanger system. The heat generated by the burners within the firebox of the furnace provides radiant heat for the cracking operation. The pairing of the inlet and outlet legs of the plural furnace tubes provides for a more uniform temperature profile within the firebox, which lessens the likelihood of localized spot overheating of a tube portion that would promote coking and tarring thereat, and further enhances the thermal efficiency of furnace operations.
The cool inlet-hot outlet leg pairing of the furnace tubes of this invention differs in many beneficial respects from prior designs wherein cool inlet legs are grouped in spacings of one to another and hot outlet legs are grouped in spacings of one to another and the inlet bank of legs is widely spaced from the outlet bank of legs. With the cool inlet-hot outlet leg pairing of this design, as noted, an essentially uniform spacing exists between all legs of the multiple furnace tubes. As noted, this uniformity of leg spacing maximizes the utilization of the radiant heat which is available within the firebox and also promotes the more uniform radiant heating of each individual U-coil tube of the multiple furnace tubes. Also, this design provides for a greater concentration of tubes within the volume of space available within the firebox, meaning a greater rate of product production as a unit of firebox volume or as a unit of the heat duty for operation of the firebox. Further, product yield is more optimum since each furnace tube, being more uniformly heated, produces therein a more uniform conversion of the hydrocarbon feed therethrough to the design product. Accordingly, with design of this invention there results a cracking furnace the operation of which produces a greater production of product of more optimum product profile with an attendant greater availability and run length time for furnace operation.